Prior to the present invention, it has been known to use oxidation-reduction potential ("ORP", or redox potential) for the control of cyanide destruction by chloride oxidation in blast furnace gas washing effluent. See Myatt, Aston and Johnson, 46 Iron & Steel International No. 5, October, 1973. It has also been known to use breakpoint chlorination for ammonia-nitrogen (NH.sub.3 --N) removal from wastewater. See EPA Publication 670/2-73-058, September, 1973 "Ammonia-Nitrogen Removal by Breakpoint Chlorination", by Pressley, Bishop, Pinto and Cassel, and related U.S. Pat. Nos. 3,732,164; 3,733,266, and 3,760,829.
U.S. Pat. No. 3,920,419 describes the removal of ammonia from ammonia-containing liquor by a process that consists of automatically adjusting the pH of the liquor by the continuously controlled addition of sodium hydroxide (NaOH) solution in sufficient quantity to maintain the pH at a minimum value of 10.5, stripping ammonia from the liquor by passing a counter-current stream of air through a packed column at a temperature of 60.degree. to 82.degree. C. (140.degree. to 180.degree. F.), and controlling the A/L ratio over the preferred range of 0.37 to 0.74 cubic meters per liter (m.sup.3 /l) (50 to 100 cubic feet per gallon) so that at least 99% of the ammonia is removed from the liquor. The application of such a process to blast furnace blowdown would require large quantities of energy, especially during the winter months, to warm the blowdown and the stripping air and, thus, to maintain the recommended column temperature. The relative volatility of ammonia may be affected by the various other components dissolved in the blowdown. Finally, because the blowdown NH.sub.3 --N concentration of blast furnace wastewater may be expected to be approximately 100 mg/l, whereas the patented process describes stripping ammonia liquors containing 4000 to 6500 mg/l, the claimed nitrogen removal efficiency of at least 99% would probably not apply.
It is also conventional to remove organic materials of various kinds from water through adsorption on activated carbon. See U.S. Pat. No. 3,455,820 and, particularly, Bauer and Snoeyink, "Reactions of Chloramines with Active Carbon", Journal Water Pollution Control Federation, Vol. 45, No. 11, November, 1973, p. 2290.
Osantowski and Geinopolos described a two-stage chlorination-oxidation process (EPA Symposium on Iron and Steel Pollution Abatement Technology, Chicago, Ill., Oct. 31, 1979) but no data are disclosed concerning the pH at which the second stage is maintained or the method of controlling the chlorination.
So far as we are aware, however, no one has succeeded, prior to our efforts, in conducting a two-stage chlorination-oxidation process wherein the first stage is maintained at a pH greater than about 8.5 and the second stage at a pH about 6-8 (preferably 7-7.5) and wherein the chlorine addition to the first stage is controlled by the redox potential in the second stage. We have found that the chlorination process efficiently removes the nitrogen-containing compounds without the detectable formation of chlorophenols, and other organics are efficiently removed by the activated carbon; cyanides, thiocyanates, and other highly undesirable materials such as chloramines and halomethanes are also destroyed or adsorbed. If granular activated carbon is used, it may be regenerated in a known manner.
Breakpoint chlorination of municipal wastewater followed by activated-carbon treatment has been studied by a number of workers. In these studies NH.sub.3 --N concentrations were about 30 mg/l or less. The presence of other chlorine (Cl.sub.2) demanders found in blowdown from blast furnaces, particularly cyanide and thiocyanate, is not of immediate concern to persons working with municipal wastewater. The chemistry and kinetics of the reaction of aqueous ammonium systems with chlorine to yield mono-, di-, and trichloramines as a function of pH have been well documented since the work of A. E. Griffin, and N. S. Chamberlin, "Some Chemical Aspects of Break-Point Chlorination", J. NEWWA, Vol. 55, 1941, p. 371; A. E. Griffin, "Chlorine for Ammonia Removal", 5th Annual Water Conference Proc. Engrs. Soc., Western Pennsylvania, 1944, p. 27, and I. Weil and J. C. Morris, "Kinetic Studies on Chloramines", J. Amer. Chem. Soc., Vol. 71, 1949, p. 1664. There is general agreement that the steps involved in the removal of ammonia by breakpoint chlorination are EQU 1. Cl.sub.2 +H.sub.2 O.fwdarw.HOCl+HCl EQU 2. NH.sub.4.sup.+ +HOCl.fwdarw.NH.sub.2 Cl+H.sub.2 O+H.sup.+ EQU 3. 2NH.sub.2 Cl+HOCl.fwdarw.N.sub.2 +3HCl+H.sub.2 C
To favor this sequence and to avoid formation of the undesirable side products, di- and trichloramines, the reaction is controlled at a pH between 5.8 and 9.5 and preferably between 6.0 and 8.0. T. A. Pressley, et al, "Ammonium Nitrogen Removal by Break-Point Chlorination", Environ. Sci. Technol., Vol. 6, 1973, p. 622. A. T. Palin, "A Study of the Chloro Derivatives of Ammonia and Related Compounds with Special Reference to Their Formation in the Chlorination of Natural and Polluted Waters", Water and Waste Eng., 1950, p. 151, reports a pH range of 7.0 to 8.5 as being optimum for monochloramine formation.
The alkaline chlorination of cyanide-bearing wastewaters is also well known technology. It is based on the reaction of cyanide to form cyanogen chloride, which at a pH above 8.5 is hydrolyzed to cyanate. The rate of hydrolysis increases with increasing pH. At pH below 8.5, the rate is slow, and the escape of toxic cyanogen chloride to the atmosphere is of some concern. Therefore, it is evident that at pH favoring the removal of NH.sub.3 by optimum monochloramine formation, the hydrolysis of cyanogen chloride to cyanate is suppressed. This point becomes a consideration in the chlorination of blast-furnace blowdown that contains both these contaminants.
The efficiency of activated carbon to remove residual chlorine, chloramines and organics from chlorinated wastewater has been studied. See P. F. Atkins, et al., "Ammonia Removal by Physical-Chemical Treatment", Jour. Water Poll. Control Fed., Vol. 45, No. 11, 1973, p. 2372; R. C. Bauer and V. H. Snoeyink, "Reactions of Chloramines with Active Carbon", Jour. Water Poll. Control Fed., Vol. 45, No. 11, 1973, p. 2291; V. Magee, "The Application of Granular Activated Carbon for Dechlorination of Water Supplies", Proc. Soc. Water Test. and Exam., Vol. 5, 1956, p. 17; D. G. Hager and M. E. Flentye, "Removal of Organic Contaminants by Granular Carbon Filtration", Jour. Amer. Water Works Assn., Vol. 57, 1965, p. 1440; A. W. Lawrence, et al., "Ammonia-Nitrogen Removal from Wastewater Effluents by Chlorination", Paper presented at 4th Mid-Atlantic Industrial Waste Conf., Univ. of Delaware, Newark, Del. (Nov. 19, 1970); and R. A. D'Agostaro, "An Investigation into the Mechanisms of Nitrogen Removal from Aqueous Solution Utilizing Chlorination Followed by Activated Carbon Contact", M. S. Thesis, Cornell University, Ithaca, N.Y., 1972 found some of the chloramines to be reconverted to NH.sub.3 upon carbon treatment. Atkins and co-workers, P. F. Atkins et al, "Ammonia Removal by Physical-Chemical Treatment", Jour. Water Poll. Control Fed., Vol. 45, No. 11, 1973, p. 2372, found that carbon treatment eliminated all chloramines and free chlorine and left the effluent essentially void of residual chlorine.